Twisted pairs of copper telephone wire, commonly referred to as twisted pairs, have been in existence since the late nineteenth century when Alexander Graham Bell first invented them. In fact, most of the transmission lines in the U.S. telephone network, commonly referred to as the telephone loop plant, are twisted pairs. Although much of the loop plant has been or is currently being upgraded with optical fiber, it is expected that twisted pair technology will remain in place for many years due to the high cost of upgrading and due to the ability to achieve high transmission rates over twisted pairs.
Digital subscriber line (DSL) technology, first developed to support basic rate integrated services digital network (ISDN) transmission over twisted pair, has made it possible to achieve high transmission rates of video, audio and data over twisted pairs. Today, there are many variations of DSL technology in use, such as asymmetric digital subscriber line (ADSL), high-bit-rate DSL (HDSL), and rate-adaptive DSL (RADSL), which are collectively classified under the rubric "xDSL". All of these technologies correspond to line coding schemes which can be used to transmit and receive voice, video, and data over existing copper twisted pairs.
Twisted pairs attenuate signals in proportion to the length of the line and the frequency. In order to compensate for this characteristic, telephone companies design lines in accordance with certain standards designed to maximize performance, such as requiring that wires which extend over certain distances be a certain gauge and requiring that loading coils be installed on lines in many cases. Although, attenuation generally is the dominant factor affecting performance of twisted pairs, cross talk also affects performance. Cross talk corresponds to interference in a twisted pair caused by an adjacent twisted pair. Cross talk increases with frequency and with the number of cross-talking pairs. ADSL was developed to reduce the effects of cross talk. In accordance with ADSL technology, data is transmitted upstream at one rate and downstream at a much higher rate than the upstream rate, with the upstream and downstream data being separated through the use of frequency division multiplexing.
Another factor which affects the performance of twisted pairs is near and far end echo resulting from impedance differences created by telephone loop hybrid circuits. Telephone loop hybrid circuits are necessary in order to enable full duplexing of the signals transmitted between the end user and the central office. Echo cancellation techniques allow the echo to be calculated and canceled out to improve signal quality. Echo cancellation technology has now been widely implemented and most modems manufactured today contain an echo cancellation component.
Another factor which affects the performance of twisted pairs is noise or interference created by sources which are external to the telephone network. This interference may emanate from local area sources, such as electrical wiring within the customer premises, or from broad area sources, such as power lines which run along side the telephone lines within the loop plant and from radio transmitting stations. It would be beneficial to provide a method and apparatus capable of eliminating or reducing local area and broad area interference in a twisted pair transmission system in order to maximize performance of twisted pairs.